1. Field of the Invention
The present invention relates generally to an improvement in a fuel injection control system for an internal combustion engine to control fuel injection amount in accordance with engine operating conditions, and more particularly to such a fuel injection control system arranged to decide an appropriate fuel injection amount during transient time or transient engine operation (such as acceleration and deceleration) of engine operation by correcting a standard fuel injection amount in accordance with engine operating conditions.
2. Description of the Prior Art
In connection with fuel injection control by using a fuel injection control system for an automotive internal combustion engine, shift of air-fuel ratio of air-fuel mixture from a target level generally largely depends upon change in amount of fuel adhering on the inner wall surface of an intake manifold and an intake port of an intake system of the engine and fuel floating in the same places. The amount of the adhering and floating fuel changes largely depending upon engine operating conditions. Furthermore, the amount of such adhering and floating fuel does change stepwise but changes with delay whose time constant is variable. Moreover, change in the amount of the adhering and floating fuel greatly depends not only upon engine operating conditions but also upon the difference between the amount of adhering and floating fuel at that point of time and that in an equilibrium state (steady state). Thus, the amount of the adhering and floating fuel in the intake system changes in a very complicated mechanism during engine operations and therefore it is difficult to control fuel injection amount precisely in accordance with engine operating conditions, particularly during transient time of engine operation.
In order to attain precise fuel injection control, a proposal has been made as disclosed in European Patent Publication No. 0152019 (Application No. 85100998.5). This proposal is directed to a method for controlling fuel injection for an engine in which, on the basis of a phenomenon that a part of fuel vapored from a liquid film adhered on a wall surface of an intake manifold remains in an intake manifold in the form of fuel vapor, the quantity of the liquid film and the quantity of the fuel vapor are estimated by using control parameters such as air mass flowing through a throttle valve, a throttle opening degree, an engine speed, an air-fuel ratio, etc. The quantity of the liquid film and the quantity of the fuel vapor at a desired point of time are predicted on the basis of the result of estimation. Additionally, the quantity of fuel injection is controlled so as to make the air-fuel ratio be a desired level. Further, the quantity of the liquid film is estimated in the case where the data as to the air-fuel ratio obtained by an O.sub.2 sensor includes an observation delay. A sum of the quantity of fuel vapored from the liquid film at a desired point of time and the quantity of fuel which does not adhere on a wall surface of the intake manifold is predicted on the basis of the result of the estimation. Additionally, the quantity of fuel injection is controlled so as to make the observed air-fuel ratio be a desired lever on the assumption that the quantity of fuel corresponding to the estimated sum is sucked into an engine cylinder.
However, in such a conventional fuel injection control method, transient time of engine operation have been intensively taken into consideration and therefore correction coefficient for the transient time has not decided. Accordingly, with this conventional fuel injection control method, it is impossible to achieve a precise fuel injection control in accordance with engine operating conditions, particularly during transient time of engine operation.